The present invention relates to an eye examining instrument, particularly one for exactly focusing on the retina of a human eye.
Generally, to focus an eye fundus camera, the eye fundus is illuminated, the diopter of the observer is matched and a cross line on a focusing screen, the clear image of the eye fundus is obtained. However, the fundus of the human eye contains few focusing targets. When the veins in the fundus are used as a target, the optical axis of the camera has to be deflected to bring the veins into the visual field and then the camera returned to its initial visual field. This is troublesome and requires substantial experience.
In order to eliminate these difficulties, U.S. Pat. No. 3,016,000 proposes a system in which the image of a small cross line is projected on the eye fundus and served directly or through a half-transparent mirror. The system is then focused to obtain a clear image of the cross line. However, the individual differences play an important role in the judgement of the clearness of the image.
On the other hand, U.S. Pat. No. 3,925,793 proposes an eye fundus camera focus which eliminates individual differences. However, this camera focuses by making a focusing target is out of the paraxis and the focusing accuracy is unavoidably decreased.